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Interim Recommendation issued to: Civil Aviation Safety Authority

Recommendation details
Output No: IR19990115
Date issued: 13 August 1999
Safety action status:
Background: Why this Interim Recommendation was developed

Output text

The Bureau of Air Safety Investigation recommends that the Civil Aviation Safety Authority Australia conduct an audit of that part of the aviation safety system that establishes the operational history of life-limited components, to establish why the operational history of second-stage turbine wheel, p/n 868272-1, s/n P03214C, could not be determined with certainty.

As a result of investigation into this occurrence, the Bureau simultaneously issues the following interim recommendations and safety advisory notice:


IR19990112

The Bureau of Air Safety Investigation recommends that the engine manufacturer AlliedSignal audit the process employed to manufacture TPE 331 turbine wheel knife-edged seals to determine those factors that may lead to excessive variations in slot corner radii.


IR19990113

The Bureau of Air Safety Investigation recommends that the engine manufacturer AlliedSignal determine the sensitivity of turbine wheel seals to the initiation of fatigue cracks from slot corners, as a function of slot corner radii.


IR19990114

The Bureau of Air Safety Investigation recommends that the engine manufacturer AlliedSignal consider, during engine design and the formulation of continuing airworthiness instructions, the possibility that turbine wheel seal fatigue failure may result in hazardous modes of engine failure.


SAN19990116

The Federal Aviation Administration should note the safety deficiency identified in this document and take appropriate action as considered necessary.

Initial response
Date issued: 03 December 1999
Response from: Civil Aviation Safety Authority
Action status: Open
Response text:

CASA has reviewed Air Safety Interim Recommendation IR 999015 (sic). The cause of the failure is agreed to be rubbing wear of the wheel near the transition from hub to web. This wear is believed to have occurred from contact with the second stage seal. BASI has advanced a reason for failure of this seal, but we note that the reason advanced elsewhere by the engine manufacturer differs. Any further advice from the manufacturer, subsequent to issue of your report, would be appreciated.

The recommendations in the report are agreed, and CASA will review the regulatory requirements related to recording the operational history of life limited components.

ATSB response:

The following correspondence was forwarded to the Civil Aviation Safety Authority on 3 November 1999:

I refer to your letter BE99/253 dated 25 October 1999 in which you requested further advice the ATSB may have from the manufacturer, AlliedSignal in relation to the ATSB's safety recommendations relating to the Pel Air TPE-331 uncontained engine failure.

The ATSB has received information from AlliedSignal stating that they agree that the fracture of the seal fitted to the second stage turbine wheel caused the fracture of the wheel and the release of fragments that penetrated the engine nacelle and aircraft fuselage.

The cause of the seal fracture has not been determined with certainty. ATSB analysis identified fatigue cracking as a possible cause, with variations in the size of slot corner radii acting as a local stress raiser. AlliedSignal consider that an increase in operating temperature of the seal was the cause of fracture. They base this hypothesis on the presence of blockages in the cooling air flow holes in the second stage stator asembly (sic). The blockages are attributed to an improper repair of the second stage stator.

It appears that AlliedSignal consider that the overheating of the seal resulted in an overstress fracture. It is possible that engine operation over a number of flights, with the seal being heated to a higher than normal temperature, may lead to thermal fatigue.

At present there is a lack of hard evidence to support either hypothesis.

In recognition of the lack of evidence, AlliedSignal, in response to the recommendations, have agreed to perform a sensitivity study on turbine wheel knife-edged seals with respect to fatigue crack initiation life. The analysis will be completed by March 2000. They have also specified in-service inspection requirements for the seal. Fluorescent penetrant inspections of the second stag turbine assembly include the seal. The inspection acceptance criteria is "no cracks allowed".

AlliedSignal will inform the ATSB of the results of the testing program.

In the last paragraph of your letter you state that CASA will review the regulatory requirements related to recording the operational history of life limited components. With respect to this review, please advise when the ATSB can expect the results.

Further correspondence
Date issued: 03 December 1999
Response from: Civil Aviation Safety Authority
Response status: Closed - Accepted
Response text:

An audit of the subject matter has identified a need to review Civil Aviation Regulation (1988) 42W (5)(iii). The regulation requires that time in service or the number of cycles be recorded. The Certificate of Approval holder who installed the second-stage turbine wheel had complied with CAR 42W in regard to the recording of time in service. On this occasion, the recording of merely the time in service meant that the operational history of the subject turbine wheel could not be determined with certainty.

The matter has been raised with CASA Maintenance Standards Group, Airworthiness Branch, for review and action as necessary.

 
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Last update 01 April 2011